First star can be found in the hub at the bottom. First you must use the axe to unlock the blue transmitter on the left side of the tower. Then, get two connectors from the first floor and throw them to the bottom.
Link two connectors to unlock the door before the star in a remote building. Take the penultimate star.
Last star can be unlocked from the fifth floor. Walk outside on thin bars near the wall. Walk down through them.
Then, when jumping down you must aim at the hole in the roof and get inside the tower. There you will find the last star.